The present invention relates to bridges for providing a connection between local area networks, and in particular to a bridge that provides a connection between a MODBUS Plus network and an Ethernet network.
Local area networks are widely used for communicating between data processing systems and peripherals such as, for example, those disclosed by U.S. patent application Ser. No. 09/036,565, filed Mar. 9, 1998, now U.S. Pat. No. 6,016, 523 and incorporated herein by reference, and German Patent No. DE 196 15 093 A1, filed Apr. 17, 1996, and also incorporated herein by reference.
As is well known by those skilled in the art, local area networks use at least one of many network protocols for passing data along the network. Such widely used network protocols include the MODBUS Plus Protocol and the Ethernet Transport Control Protocol/Internet Protocol (TCP/IP).
The MODBUS Application Protocol used for sending MODBUS messages over TCP/IP. The MODBUS Application Protocol (MBAP), is a layer-7 protocol providing peer-to-peer communication between Programmable Logic Controllers and other host-based nodes on a local area network. Collectively, these nodes implement all or part of a control application widely used for industrial applications in the automotive, tire and rubber, food and beverage, and utilities industries to name a few.
MODBUS protocol transactions are typical request-response message pairs. MODBUS requests contain function codes representing several classes of service including data access, online programming, and program download and upload classes. MODBUS responses can be acknowledgments with and without data, or non-acknowledgments with error information. The MODBUS protocol is described in a reference manual entitled MODBUS Protocol Reference Guide, publication PI-MBUS-300, incorporated herein by reference.
Because of the widespread use of MODBUS Plus and TCP/IP Ethernet networks, there is a need to have these networks communicate with each other. To the knowledge of the inventors, no bridges are available for facilitating such communication path(s). Accordingly, the present invention satisfies this need.
The inventive bridge disclosed herein provides a transparent multipath connection between Ethernet host applications and a MODBUS Plus industrial network. The bridge functions as an addressable node on each network, managing the Ethernet and MODBUS Plus protocols and translating messages between the network applications in both directions.
Ethernet hosts can establish connections to the bridge and send messages intended for MODBUS Plus nodes. The bridge accepts the messages, creates internal paths for the MODBUS Plus transactions, waits for the MODBUS Plus token frame, and forwards the messages to the destination nodes. It receives the data responses from the MODBUS Plus nodes, returns them to the Ethernet hosts, and closes the paths.
Messages sent to the bridge on either Ethernet or MODBUS Plus contain an embedded MODBUS message, consisting of a function code and additional information such as register addresses and data. The bridge uses the MODBUS function code to determine the type of internal path required for the transaction, and opens a path if one is available. If a path is not available, a MODBUS error code (exception code) is returned by the bridge to the requesting node.
The bridge provides MODBUS Data Master, Data Slave, Program Master and Program Slave paths as defined by the MODBUS function codes in the messages. In one embodiment, the bridge supports up to eight concurrent Data paths and eight concurrent Program paths. Moreover, the bridge supports full five-byte MODBUS Plus routing, enabling transactions through MODICON BP85 MODBUS Plus bridges to MODBUS Plus nodes on up to three networks beyond the Ethernet bridge.
The bridge maintains two internal mapping tables that provide address mapping between Internet Protocol addresses (i.e., 255.255.255.255) and MODBUS Plus path addresses (i.e., 64.64.64.64.64). The user can define the mapping at a local keyboard and monitor connection, using a configuration utility that is stored in the bridge. Mapping tables can also be downloaded by Hypertext Transfer Protocol (HTTP) from the Ethernet host to the bridge. If neither method is used, the bridge provides default mapping tables.
The user can assign the bridge""s Internet Protocol (IP) address through a Bootstrap Protocol (BOOTP) server using the bridge""s Media Access Control (MAC) address, or locally by means of the bridge""s configuration utility. The bridge""s MODBUS Plus address is assigned in hardware switches on the bridge""s MODBUS Plus circuit board.
The bridge can be mounted on a vertical panel or horizontal shelf. Its rear panel has connectors for the networks and for operating power. Also, light emitting diode (LED) indicators are provided for power, Ethernet communication, and MODBUS Plus communication status.